Generations of sequences of waveforms

ABSTRACT

A sequence of waveforms is generated by producing a primary symbol sequence and, for each symbol within the sequence, substituting a randomly-selected waveform. The primary symbol sequence has a narrow autocorrelation function, and may be a train of pulses, with the pulses arranged in packets of predetermined configuration. Objects can be detected by transmitting the waveforms, forming a representation of the transmitted waveform and delaying that representation, and correlating the representation with signals received as a result of reflection of the transmitted waveforms.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for generatingsequences of waveforms, for example to be utilized in obstacle-detectionsystems and particularly, but not exclusively, in automotive blind-spotor pre-crash warning systems designed to operate in multi-userenvironments.

BACKGROUND OF THE INVENTION

One important type of automotive blind-spot or pre-crash warning systememploys short pulses of electromagnetic or ultrasonic energy tointerrogate the detection zone. A decision regarding the presence orabsence of an obstacle at a predetermined range is then made by suitablyprocessing energy backscattered by various objects in the field of viewof the system. A detection apparatus can be arranged to detect whether areflected pulse appears at a particular delay (corresponding to thepredetermined range) after the interrogation pulse has been transmitted.Instead of just detecting the occurrence of reflected pulses, the analogvalues of the signal received after each pulse transmission can beintegrated, to provide an arrangement which is sensitive tobackscattered signal strength, and thus capable of better performance. Amultichannel detection apparatus has several channels, each with adifferent delay, for detecting objects at different range values ofinterest.

It is known that object detectability can be improved significantly wheneach single pulse is replaced by a suitably-constructed pulse packet.Each pulse packet comprises a specified number N of identical pulseswhich are staggered nonuniformly, with each interpulse spacing being aninteger multiple of a suitably chosen unit time interval. The pattern ofinterpulse spacings is so designed as to ensure that only a small numberha of pulse coincidences (preferably at most one pulse coincidence) willoccur between a primary pulse packet and its replica shifted in time bymore than one pulse duration. This condition is usually referred to asthe autocorrelation constraint.

Consider a pulse packet of span (length) L comprising N identicalrectangular pulses of unit duration. Such a pulse packet can beconveniently represented by a binary sequence {x}=x₁ x₂ . . . x_(L) ofsymbols 0 and 1, in which symbol 1 corresponds to pulse occurrence. Inthis case, the autocorrelation constraint can be expressed as${{R_{xx}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}x_{i + d}}} \leq h_{a} < N}},\quad{0 < d \leq {L - 1}}$where R_(xx)(d) is the autocorrelation sequence and d is the integershift. When d=0, the autocorrelation value R_(xx)(0) simply equals thenumber N of pulses contained within the pulse packet.

In the class of all pulse packets with a specified number of pulses Nand h_(a)=1, a maximally compact pulse packet has the minimal spanL_(min). Consequently, the maximally compact pulse packet exhibits thelargest duty factor, N/L, hence the largest average power. For a fixed Nand h_(a)=1, all pulse packets with spans greater than Lin are referredto as sparse pulse packets.

The autocorrelation constraint ensures that when there is no noise orinterference, and a multichannel pulse-coincidence processor is used fordetecting a pulse packet, the output of each channel is at most haexcept when the channel delay matches that of a pulse packet beingreceived. In this case, the channel output reaches the peak value of N.

In a multi-user environment, the users may transmit their signalssimultaneously and asynchronously so that not only must each receiverrecognize and detect its own transmitted signal, but it must be able todo so in the presence of the other transmitted signals. Assume that apulse packet to be detected by a receiver of interest is represented bya binary sequence{x}=x₁ x₂ . . . x_(L)and that one of the interfering pulse packets is represented by anotherbinary sequence{y}=y₁ y₂ . . . y_(L)

In order to optimize the detection performance of the receiver inmulti-user environment, the following cross-correlation constraints mustbe satisfied for all integer shifts d${{R_{xy}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}y_{i + d}}} \leq h_{c} < N}},\quad{0 \leq d \leq {L - {1\quad{and}}}}$${{R_{yx}(d)} = {{\sum\limits_{i = 1}^{L - d}{y_{i}x_{i + d}}} \leq h_{c} < N}},\quad{0 \leq d \leq {L - 1}}$

When more than one transmitter is in operation, the autocorrelation andcross-correlation constraints combined together ensure that when thereis no noise and a multichannel pulse-coincidence processor is used fordetection, the output of each channel is still substantially less than Nexcept when the channel delay matches that of a received pulse packet ofinterest.

In automotive applications, many similar obstacle-detection systemsshould be capable of operating in the same region, and sharing the samefrequency band. To avoid mutual interference, each system should use adistinct signal, preferably uncorrelated with the signals employed byall other systems. Because it is not possible to predict which of themany similar systems will be operating in a particular environment, itis not practical to assign a distinct binary sequence to each of them.Furthermore, it is also very difficult to construct large sets of binarysequences with good autocorrelation and cross-correlation properties,and also exhibiting acceptable duty factors.

European Patent Application No. EP-A-1330031 (referred to herein as “theearlier application”, and the contents of which are incorporated hereinby reference) discloses a method which exploits random mechanisms togenerate large sets of composite pulse trains that can satisfy bothautocorrelation and cross-correlation constraints. Consequently,resulting composite pulse trains will exhibit an excellent resistance tomutual jamming in multi-user environments. It is also possible toproduce sequences exhibiting a high duty factor, thus enhancing theresulting detection performance.

According to the method disclosed in the earlier application, acomposite pulse train consists of a sequence of primary pulse packetseach of which is drawn at random from a predetermined set of suitablyconstructed primary pulse packets with prescribed autocorrelation andcross-correlation properties. The autocorrelation function of eachprimary pulse packet exhibits the property of ‘at most one coincidence’.Also, the cross-correlation function between any two different pulsepackets assumes small values compared to the maximum value of thecorresponding autocorrelation functions. Furthermore, the resistance tomutual jamming in multi-user environments can be further improved byseparating individual primary pulse packets by gaps of random duration,the value of which may be determined by a random value supplied by arandom number generator. FIG. 1 depicts the structure of suchconstructed composite pulse trains.

As a result, although each user may have the same set of primary pulsepackets, a composite pulse train transmitted by each user is assembledin a random manner and is, therefore, statistically unique.

Although the method disclosed in the earlier application offers apractical solution to the problem of alleviating the mutual interferenceeffects in a multi-user environment, the method is not capable ofincreasing the ratio R of the peak autocorrelation value, R_(xx)(0)=N,to the maximum (unit) autocorrelation sidelobe value. Increasing thevalue of R would improve the capability of the obstacle-detection systemto detect and discriminate smaller obstacles (such as motorbikes)located in proximity of larger obstacles (such as trucks).

It would therefore be desirable to provide a method for generating alarge number of pulse trains with good autocorrelation properties, goodcross-correlation properties, and also improved capability todiscriminate between multiple smaller and larger obstacles, especiallyfor applications in systems intended to operate in a multi-userenvironment.

DESCRIPTION OF THE INVENTION

Aspects of the present invention are set out in the accompanying claims.

In accordance with another aspect of the invention, a primary symbolsequence having a narrow autocorrelation function (i.e. anautocorrelation function which, for all non-zero shifts, has a valuewhich is substantially smaller than the maximum value at zero shift) ischosen, and each symbol of the sequence is replaced (substituted) by awaveform, drawn at random from a set comprising a finite number ofsuitably chosen waveforms of finite duration. Preferably, the waveformsshould have the same duration and be mutually orthogonal (uncorrelated)to facilitate their discrimination in the receiver.

For clarity, the invention will primarily be described in the context ofarrangements wherein the primary symbol sequence is a train of discretepulses, and the pulses are arranged in predetermined packets (althoughneither of these features is essential, as will be described). Themechanism of representing a pulse by a randomly selected waveform may beviewed as some form of random pulse mapping.

When M orthogonal waveforms are utilized for pulse mapping, a singlepulse packet containing N pulses may be represented by as many as M^(N)waveform packets, all packets conveying the same time information, yetbeing different, hence distinguishable in the receiver.

The operation of random pulse mapping retains the time informationcontained in the pulse-to-pulse intervals, and random pulse mappingitself is equivalent to assigning to each pulse an index selected atrandom from a set of integers. Those indices may be represented in aphysical system in a variety of different ways (for example, by thefrequencies of the waveforms).

When no random mapping is applied to a pulse packet comprising N pulseswith the property ‘at most one coincidence’, the ratio R of the peakautocorrelation value R_(xx)(0) to the maximum (unit) autocorrelationsidelobe value is just N. However, when M orthogonal waveforms areutilized for random pulse mapping, the unit sidelobe value will, onaverage, be reduced to 1/M. This effect follows from the fact that therequirement of pulse coincidence ‘in time’ is now combined with therequirement of equality of indices (additional coincidence) assigned toindividual pulses. As a result, the average value of the ratio R will beincreased to NM, and an improved discrimination between large and smallobstacles will be obtained.

Random pulse mapping, even if used on its own, provides an excellentresistance to mutual jamming in a multi-user environment. Byconstruction, although each user may employ repeatedly the same primarysymbol sequence (e.g. a primary pulse packet), the correspondingsequence of waveforms is assembled in a random manner by each user andis, therefore, statistically unique. The resistance to mutual jamming ina multi-user environment can be further improved by inserting randomgaps between individual packets in accordance with the method describedabove and disclosed in the earlier application. The techniques disclosedin GB-A-2357610 could be used to determine the durations of the gaps.

In some obstacle-detection systems, the peak power of transmittedwaveforms is limited and cannot be increased, yet attaining reliabledetection coupled with high range resolution is of primary importance.In such cases, a set of orthogonal waveforms used for random pulsecoding should contain waveforms which can be compressed in the receiver,and their duration on transmit could be increased up to (Z+1)Δ, where ZΔis the shortest interval between primary signals in a packet. The use of‘compressible’ waveforms for random pulse mapping will increase theresulting duty factor, by a factor of up to (Z+1), which otherwise maybe too low for intended applications.

A broad class of compressible waveforms include sine waves, andtransient signals with linear frequency modulation, commonly referred toas LFM, or ‘chirp’, pulses, and other waveforms known to those skilledin the art.

As described in the earlier application, a primary pulse packet withdesired autocorrelation properties can be used to construct anotherprimary pulse packet with the same autocorrelation properties byreversing in time the first primary pulse packet. The cross-correlationfunction between these two dual primary pulse packets will not exceedvalues greater than two. In a preferred aspect of the present invention,a pulse packet with a larger duty factor may be formed by basing thepacket on an underlying binary pulse sequence formed by suitablyinterleaving a first pulse packet and its time-reversed (‘mirror’)replica. In such a case, preferably, one set of waveforms will beemployed for mapping the first pulse packet, and another set ofwaveforms will be used for mapping the ‘mirror’ replica of the firstpacket. Preferably, the two sets of waveforms are mutually exclusive,i.e., no waveform may belong to both the sets. As a simple example, thewaveforms used for the primary packet may have a first frequency, andthose for the mirror replica may have a second frequency. It ispreferred, however, for each packet to comprise several differentwaveforms.

According to a further preferred aspect of the invention, a pulse packetis constructed in such a way as to satisfy the autocorrelationconstraint modified as follows${{R_{xx}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}x_{i + d}}} = 0}},\quad{0 < d \leq Z}$${{{R_{xx}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}x_{i + d}}} \leq 1}},\quad{Z < d \leq {L - 1}}}\quad$where the pulse packet is represented by a binary sequence {x₁ x₂ . . .x_(L)} of symbols 0 and 1, in which symbol 1 corresponds to pulseoccurrence.

The above constraint is more restrictive than the ‘at most onecoincidence’ requirement, because for all consecutive delay values d=1,2, . . . , Z, the autocorrelation sequence R_(xx)(d) must equal zero.Therefore, the resulting autocorrelation sequence R_(xx)(d) will exhibita zero-correlation zone of span Z on either side of the main peak ofvalue N at d=0.

When the duration of a single pulse equals Δ, the zero-correlation zonewill correspond to the relative distance ZCD between obstacles equal toZcΔ/2, where c is the speed of interrogating pulses (c is the speed oflight for electromagnetic pulses).

Consequently, an obstacle-detection system utilizing pulse packetssatisfying the modified autocorrelation constraint will have an improvedobstacle resolution, because the sidelobes of the autocorrelationfunction corresponding to a larger obstacle (such as truck) will nolonger obscure the main autocorrelation peak associated with a smallerobstacle (such as motorbike) located within the relative distance ZCDfrom a larger obstacle.

From the modified autocorrelation constraint, it follows that in orderto obtain the zero-correlation zone of span Z, the minimum differencebetween any two pulse positions in the pulse packet cannot be less than(Z+1).

Using this aspect of the invention, the improved discrimination betweenlarge and small obstacles achieved as a result of random pulse mappingis further enhanced.

Combining the aspects mentioned above, i.e. the modified autocorrelationconstraint (leading to an extended zero-correlation zone) and thestatistical sidelobe reduction, obtained from random pulse mapping,results in the following set of packet properties:1. autocorrelation peak N${R_{xx}(0)} = {{\sum\limits_{i = 1}^{L}x_{i}^{2}} = N}$2. zero-correlation zone${{R_{xx}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}x_{i + d}}} = 0}},\quad{0 < d \leq Z}$3. statistical sidelobe reduction via random pulse mapping${{{R_{xx}(d)} = {{\sum\limits_{i = 1}^{L - d}{x_{i}x_{i + d}}} \leq {1/M} < 1}},\quad{Z < d \leq {L - 1}}}\quad$where the statistical bound 1/M on the sidelobe values can be satisfiedin the ‘long run’ or ‘on average’. Since in practical applications adecision regarding the presence or absence of an obstacle at a givenrange may be based on several thousand pulse packets, the bound 1/M iseffectively achievable.

The primary symbol sequence may comprise identical pulses interspersedwith gaps. In this case, the timing of the pulses is such that a narrowautocorrelation sequence is obtained. Alternatively, the primary symbolsequence may comprise different types of symbols (e.g. a binary sequencein which symbols −1 and 1 represent different polarity pulses, or aternary sequence, etc.). In this case, the autocorrelation sequence mayhave a narrow structure due to the use of distinctive symbols, inaddition to the timing of those symbols. Accordingly, to preserve thisinformation, the random pulse mapping preferably uses different sets ofwaveforms for the different symbol types, the waveforms of each setgenerally being distinguishable from those of the other sets. A specificexample of this possibility is the interleaving of a pulse packet withits ‘mirror’-reversed replica, mentioned above.

DESCRIPTION OF THE DRAWINGS

An arrangement embodying the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 depicts the structure of a composite pulse train;

FIG. 2(a) is an example of a pulse packet;

FIG. 2(b) depicts the autocorrelation sequence of a binary sequencerepresenting the pulse packet of FIG. 2(a);

FIG. 3 illustrates the principle of random pulse mapping;

FIG. 4 is an example of a pulse packet with increased duty factor;

FIG. 5 is a block diagram of an obstacle-detection system;

FIG. 6 is a more detailed block diagram of one implementation of thesystem of FIG. 5 incorporating a random pulse mapper arranged to operatein accordance with the present invention;

FIG. 7 is a block diagram of a pulse packet generator of the system ofFIG. 6; and

FIG. 8 shows different types of primary symbol sequences.

DESCRIPTION OF A PREFERRED EMBODIMENT

The structure of a system according to the present invention will bedescribed below. First, reference will be made to the types ofinterrogation signals which can be used in such a system.

FIG. 2(a) shows an example of a primary pulse packet which could be usedto derive interrogation signals used by an obstacle-detection systemaccording to the present invention. The packet comprises 11 pulsesdistributed within 106 positions; the minimum difference between pulsepositions in the packet is equal to 6 (positions 78 and 84). FIG. 2(b)depicts the autocorrelation sequence of the packet. The packet satisfiesthe modified autocorrelation constraint mentioned above. In accordancewith the packet construction, the zero-correlation zone has a span of 5.

Randomly selected waveforms are substituted for the individual pulses.In the simplest arrangement, the duration of each waveform used forrandom pulse mapping equals Δ, i.e., the duration of each primary pulse.

FIG. 3 illustrates a primary pulse packet (the same as already shown inFIG. 2(a)) and a resulting waveform packet obtained by selecting one offour available waveforms, w1(t), w2(t), w3(t) and w4(t), indexed bynumbers 1, 2, 3 and 4. In this case, the waveforms are significantlylonger than the pulses of the basic pulse packet. For illustrativepurposes, it is assumed that a random mechanism employed for waveformselection has produced the following sequence of numbers: 1, 4, 1, 2, 3,1, 3, 3, 4, 2, 4. As seen, the primary pulse packet and the resultingwaveform packet, being aligned in time, both contain the same timeinformation.

In order to improve the duty factor, the basic pulse packet can bemodified. As an illustrative example, FIG. 4(a) shows the a first pulsepacket which is the same as the primary pulse packet of FIG. 2(a), FIG.4(b) shows a ‘mirror’ replica of the packet, and FIG. 4(c) depicts apulse packet resulting from interleaving the first packet with atime-shifted ‘mirror’ replica of the packet. Again, in practice,waveforms are used in place of the discrete pulses. As indicated above,it is important for at least a substantial number of the waveformsrepresenting each packet to be distinguishable from those representingthe other packet (and desirable for them to be distinguishable from eachother). To achieve this, preferably two sets of waveforms are used,random selections from the first set being substituted for pulses of thefirst packet, and random selections from the second set beingsubstituted for pulses of the ‘mirror’ replica packet.

The duty factor of a pulse packet can be increased even more, if asupplementary packet (or packets) with the property ‘at most onecoincidence’ is inserted into the combination of a primary packet andits ‘mirror’ replica. However, more sets of waveforms will be requiredfor random mapping of pulses belonging to all interleaved packets.

FIG. 5 is a block diagram of an obstacle-detection system in accordancewith the invention. The system includes a symbol generator SG whichgenerates a sequence of symbols and provides these to a random encoderRE. The random encoder substitutes a randomly-selected waveform for eachsymbol, and delivers the waveforms in succession to an antenna driverADR coupled to a suitable transmit antenna TA.

The random decoder RE includes waveform set generators WS1, WS2, . . .WSJ, each capable of generating a respective set of waveforms. Thegenerators may be simply memories storing the waveforms. The waveformswithin each set are mutually orthogonal, and are mutually orthogonalwith respect to the waveforms in the other sets. The random encoder alsoincludes a selector SEL. The selector SEL determines a symbol valuecorresponding to the type of each symbol received from the symbolgenerator SG, and in response thereto selects a corresponding one of thewaveform stores WS1, WS2, . . . WSJ. The selector SEL also randomlyselects an index value, uses this to choose one of the waveforms storedin the selected waveform store and outputs this to the antenna driverADR.

The random encoder RE also sends information identifying therandomly-selected waveforms to a memory MEM. Preferably, the stored dataincludes the symbol value representing the symbol received from thesymbol generator SG and the random index value.

A suitable receive antenna RA is connected to an input amplifier IAM.Received signals are provided by the amplifier IAM to a decoder DECwhich determines which waveforms are received and generates index valuesrepresenting those waveforms. In a preferred embodiment, the decoder DECcomprises a number of banks of matched filters, each corresponding to arespective set of waveforms. The decoder DEC generates an index valuerepresenting the particular filter within the bank which provides anoutput, and a symbol value indicating which bank the filter belongs to.

A processor PRO receives the output values from the decoder DEC andoutput values from the memory MEM, the output values from memory MEMbeing delayed by an amount dependent on the range being investigated.These values enable the processor PRO to detect matches between thesequence of transmitted waveforms and the received signals and therebygenerate an output indicating that an obstacle has been detected.Preferably, the processor PRO checks for a match between the indexvalues provided by the memory MEM and the decoder DEC, and where thereis a match, inputs the respective symbols provided by the memory MEM anddecoder DEC to a correlator. The output of the correlator is integrated,and the resultant value is indicative of the likelihood that an objectis present at a particular range. The operation can be repeated fordifferent ranges, by using different delay values.

FIG. 6 shows a more detailed block diagram of one particular form ofobstacle-detection system. In FIG. 6, the symbol generator SG isimplemented as a pulse packet generator PPG, driven by a clock generatorCKG. The random encoder is implemented as a random pulse mapper RPM. Thememory MEM is implemented using a serial-in-parallel-out shift registerSIPO. The decoder DEC is implemented by a bank of M matched filters MF1,MF2, . . . , MFM. The processor PRO is implemented by a plurality ofrange-cell processors RCPs.

As will be explained later, the system may also incorporate anoscillator OSC supplying a sinusoidal signal at a suitable carrierfrequency to both the antenna drive ADR and the input amplifier LAM.

The pulse packet generator PPG supplies repeatedly pulses to the randompulse mapper RPM, and more specifically to: input PP of a multiplexerMPX, load input LI of a storage register SRG, and input PP of a pulseidentifier PID (which collectively correspond to the selector SEL ofFIG. 5). The system of FIG. 6 is intended for use with pulse packetscontaining identical pulses, such as shown in FIG. 2(a). Accordingly,because the pulses are identical, only one set of waveforms is required.This set is provided by M waveform generators, WG1, WG2, . . . , WGM.(To handle pulse sequences with distinguishable pulses (such as shown inFIG. 4(c), one or more additional sets of waveform generators would beprovided.)

Each pulse supplied by the generator PPG triggers one out of the Mwaveform generators in response to the value of a number generated by arandom index generator RIG. This value is loaded via input IN to astorage shift register SRG prior to the time instant of each pulseoccurrence.

Preferably, waveforms w1(t), w2(t), . . . , wM(t) to be supplied by theM waveform generators WG1, WG2, . . . , WGM should be mutuallyorthogonal to facilitate the discrimination of signals reflected back byobstacles. In particular, a suitable set of waveforms can be produced byusing short segments of sine waves, each with substantially differentfrequency. In this specific case, the bank of matched filters willcomprise band-pass filters, each with a centre frequency equal to thefrequency of a respective sine wave, and with the bandwidth inverselyproportional to the duration of the sine wave segment.

Short segments of sine waves can readily be generated by suitablymodified ringing or blocking oscillators. Another method, disclosed inU.S. Pat. No. 3,612,899 (incorporated herein by reference), enablesforming very narrow pulses of electromagnetic energy directly atmicrowave carrier frequency.

The value of a random index ID held at input OS of multiplexer MPXdetermines a specific path for each pulse to trigger the respectivewaveform generator corresponding to that index value. For example, whenthe index value is 2, waveform generator WG2 will be triggered toproduce a suitable waveform w2(t), which (via a summing amplifier SAMand driver ADR coupled to antenna TA) will be sent as an interrogatingsignal towards obstacles.

When waveforms produced by the waveform generators WG1, WG2, . . . ,WGM, are generated at a suitable carrier frequency (or frequencies),they can (after suitable conditioning and, if required, amplification indriver ADR) be sent directly by antenna TA as interrogating signals.However, when the waveform generators, WG1, WG2, . . . , WGM, can supplyonly baseband versions of waveforms, some form of ‘upconverting’(modulation) will be required prior to delivering those waveforms to thetransmit antenna TA. In such a case, the antenna driver ADR willincorporate a suitable modulator utilizing a sinusoidal reference signalat a carrier frequency, provided by the auxiliary oscillator OSC, andapplied via input CF to the driver ADR.

For each pulse provided by generator PPG, the pulse identifier PIDcombines a random index ID, assigned to that pulse (by the random indexgenerator RIG), with timing information (supplied via input PP)regarding the pulse occurrence. The resulting combination may berepresented by a binary word; for example, the most significant bit(MSB) equal 1 may mark the pulse occurrence, whereas the remaining bitsmay represent the value of the random index ID assigned to that pulse.

Binary words thus created are applied via input DI to register SIPO; thewords are shifted into register SIPO at the time instants determined byclock pulses appearing at input CP of the register SIPO. As a result,each pulse supplied by the pulse packet generator PPG is represented bya respective binary word in a unique way: the time of pulse occurrencehas been imparted on a corresponding time slot (i.e. clock period) bysetting MSB to 1, whereas the random index ID has been used to determinethe values of the remaining bits of the binary word.

The register SIPO acts as a digital discrete-time delay line with totaldelay (expressed in units of clock period) equal to the number of usedstorage cells, i.e. W in the arrangement shown in FIG. 6. Consequently,the register SIPO stores and retains the continually updated informationregarding all primary pulses generated and mapped during the last Wclock periods. This information is made available at W parallel outputsof the register SIPO; either all or only selected register SIPO outputsare connected to respective range-cell processors RCPs. For illustrativepurposes, FIG. 6 shows output K of the register SIPO connected to acorresponding processor RCP.

A signal reflected back by an obstacle and received via the receiveantenna RA is applied to the input amplifier IAM. When the matchedfilters, MF1, MF2, . . . , MFM, are capable of processing only thebaseband versions of waveforms w1(t), w2(t), . . . , wM(t), some meansfor ‘downconverting’ (demodulation) will have to be incorporated intothe amplifier IAM. Accordingly, a sinusoidal reference signal at asuitable frequency can be provided by the auxiliary oscillator OSCcoupled to input CF of the amplifier LAM.

The bank of M matched filters, MF1, MF2, . . . , MFM, is constructed tooperate as follows. When any of the utilized waveforms, w1(t), w2(t), .. . , wM(t), is applied to the common input of the matched filters, onlythe filter matched to this particular waveform will produce anunequivocal response; the residual responses of all remaining matchedfilters will be negligible. This specific property of the bank ofmatched filters is exploited to reliably recover from a received signalthe value of a random index D, assigned to each underlying pulse, duringrandom pulse mapping.

The functions and operations performed by each range-cell processor RCPcan be summarised as follows:

1. Each binary word supplied by a respective output of register SIPO isdecomposed in a word decoder WDR into a signal PP indicating the pulseoccurrence and a random index ID assigned to that pulse by the randomindex generator RIG during random pulse mapping.

2. The signal PP is applied to sampling input SS of a sampling circuitSCT, whereas the random index ID selects, via input IS of a channelselector CHS, the output of a matched filter corresponding to thatindex. Depending on the implementation, each matched filter may provideat its output either a multilevel (e.g., analog) signal, indicative ofthe strength of a received waveform, or just a binary signal, indicativeof whether or not the strength of a received waveform is substantiallygreater than that of background noise and/or interference.

3. The thus-selected output of a respective matched filter is applied tothe sampling circuit SCT and then sampled at the time instant coincidentwith the signal PP appearing at input SS. When a matched filter producesat its output a binary signal, the sampling circuit SCT can be reducedto a simple logic gate. It will, incidentally, be noted that theduration of the filter output will be dependent on the received waveformand the filter characteristics. The use of relatively long waveforms isacceptable if the characteristics are such that the filter output has asuitable duration.

4. The output of SCT is fed to an integrator INT which may be of‘integrate-and-dump’ type, or ‘running-average’ (‘moving-window’) type.When the sampling circuit SCT is replaced by a logic gate, a suitablyconfigured pulse counter can also perform the required integration.

5. The resultant level reached by the integrator INT is compared to apredetermined decision threshold DT in a comparator CMP. If the decisionthreshold DT has been exceeded, the presence of an obstacle will bedeclared in the range cell corresponding to the delay of the registerSIPO output connected to the respective range-cell processor RCP.

As seen, the main function performed jointly by the processor RCP andthe bank of matched filters is that of a waveform ‘de-mapper’ combinedwith that of a conventional correlation receiver. As a result, thedecision outputs of all processors RCPs provide a comprehensive pictureof the presence of potential obstacles in range cells constituting thefield of view (FOV) of an obstacle-detection system, from which signalsrepresenting the ranges of such objects are generated. This ‘snapshot’information can be utilized by a suitable obstacle-tracking system toproduce warning signals to alert the driver, and also other signals usedto initiate the operation of intended pre-crash actuators, such as airbags, brakes, etc. Instead of using a bank of individual range cellprocessors RCPs, a single processor RCP could be successively coupled torespective different outputs of the register SIPO so as to obtainserially decisions regarding presence of objects at respective ranges.

Where the system uses more than one type of symbol, a value representingthe type of the generated symbol would be included in each binary wordstored in the register SIPO. Also, there would be a respective bank of Mmatched filters, MF1, MF2, . . . , MFM, and associated channel selectorCHS, for each symbol type. Only (at most) one channel selector wouldprovide an output at any given time, the particular channel selectorbeing dependent on the received symbol. The collective outputs of thechannel selectors can thus generate a value representing the detectedsymbol. This would be fed to one input of a standard correlator whichreceives at its other input the symbol type value from the word decoderWDR. The output of the correlator can then be sent to the samplingcircuit SCT. This arrangement, involving multiple symbol types,facilitates improvement of auto-correlation and cross-correlationfunctions and thus improves performance.

FIG. 7 is a block diagram of one possible structure of the pulse packetgenerator PPG. The generator comprises a sequential state module SSM, astate decoder STD, a random gap generator RGG and a clock generator CKG.

During the system operation, the sequential state module SSM changes itsstate successively at the time instants determined by clock pulses CLKsupplied by the clock generator CKG. The total number NS of distinctstates of the sequential state module SSM should be at least equal tothe span L_(max) of the longest primary pulse packet used by the system;henceNS=2^(K) ≧L _(max)where K is the number of flip-flops utilized by the sequential statemodule SSM.

The sequential state module SSM is arranged to operate cyclically, eachcycle comprising NU distinct states selected in some convenient mannerfrom the total number NS=2^(K) of available distinct states. Among thoseNU distinct states, there are N predetermined states representing thepositions of pulses in each pulse packet to be generated.

The function of the sequential state module SSM can be implemented by aconventional binary counter, by a shift register with a suitablefeedback or by a similar sequential state machine well known to thoseskilled in the art.

The state decoder STD is driven by a K-bit output of the sequentialstate module SSM. The state decoder STD has two outputs: one outputsupplies a composite pulse train CPT, whereas the other produces anend-of-packet EOP pulse. For example, an end-of-packet EOP pulse maycoincide with the trailing pulse of every pulse packet. All functions ofthe state decoder STD can be implemented by a combinational logic or bya suitably programmed read-only memory.

The random gap generator RGG appends a random gap to the trailing pulseof every primary pulse packet being produced. Each cycle of therepetitive operation of the random gap generator RGG is initiated by anend-of-packet EOP pulse supplied by the state decoder STD. Theend-of-packet pulse EOP causes a random selector RS to select at randomone of a plurality of delay values stored in a delay store memory DS.The selected delay value is provided to a delay circuit DLY to cause acorresponding number of clock pulses provided by the clock generator CKGto be inhibited. The output CRG of the random gap generator RGG thussupplies a sequence of clock pulses in which a random number ofconsecutive pulses are missing. As a result, the operation of thesequential state module SSM is suspended during a random time intervalequal to the duration of the random gap. Preferably, the duration ofeach random gap is uniformly distributed, and the random gaps are formedindependently of each other.

Although the pulse packet generator PPG of FIG. 7 repeatedly generatesthe same pulse packet, as indicated in the earlier application it couldinstead be arranged such that successive pulse packets are randomlyselected from a predetermined set thereof.

The arrangement described above repeatedly produces pulse packets offinite length, and a mapping process is used to substitute waveforms forthe individual pulses. Various modifications are possible.

FIG. 8 shows a number of alternative symbol sequences which could beused in place of pulse packets. In FIG. 8(a), the primary symbolsequence is a train of non-coherent pulses, the pulses being of the same(unit) duration. The gaps between the pulses are a multiple of the unitduration, modified by varying amounts so as to obtain a narrowautocorrelation sequence. The random encoder would substitute, in placeof each pulse, a waveform randomly selected from a single set thereofFIG. 8(b) shows an alternative primary symbol sequence comprising atrain of binary pulses separated by gaps. In the arrangement shown, thepulses have values of +1 and −1. The random encoder responds to each +1pulse by selecting randomly from a first set of waveforms, and to each−1 pulse by selecting randomly from a second set of waveforms.Preferably, the decoder reconstructs a bi-polar waveform from thereceived signal, this bi-polar waveform then being subject tocorrelation with the transmitted waveform stored in the memory MEM.

As a modification, the pulse train can include pulses of more than twovalues, each different value giving rise to a selection from arespective waveform set.

FIG. 8(c) shows a further possible primary symbol sequence, in the formof discrete pulses separated by gaps, each discrete pulse containing anumber of contiguous sub-pulses (for example a Barker code) which eachsub-pulse can have one of two different values. In this case, the randomencoder will substitute a randomly-selected waveform for each sub-pulse,the waveform being selected from a set dependent upon the value of thesub-pulse.

FIG. 8(d) is an example of a primary symbol sequence in the form of acontinuous signal, in this case a pseudo-random binary sequence. Ofcourse other sequences, such a ternary or quaternary pseudo-randomsequences are alternatively possible.

In the arrangements described above, there is a fixed set of waveformsfor each symbol to be transmitted, and a random selection from this setis made each time the respective symbol is transmitted. In analternative arrangement, the set of waveforms used to represent aparticular symbol is altered (preferably at random) from time to time.

An example of this will be described with reference to the successivetransmission of a composite pulse containing a seven-element Barkersequence of sub-codes of the form:+++−−−+−

Assume that the system has at its disposal 20 waveforms: w1, w2, . . . ,w20. Prior to transmitting each composite pulse, the system selectsrandomly, e.g., three of the waveforms to represent symbol +, and threedifferent symbols to represent symbol −. For example, a system underconsideration may select w3, w12, w19 to represent symbol +, and w4, w5,w10 to represent symbol −. Information representing the selected sets isstored in a memory. Next, the system transmits the composite pulse,choosing randomly one from {w3, w12, w19} to transmit each symbol +, andchoosing randomly one from {w4, w5, w10} to transmit each symbol −.

In the receiver, the contents of the memory (after being subjected to anappropriate delay) are used to configure the outputs of the matchedfilters, such that the outputs of filters matched to w3, w12 and w19will be connected (ORed) to provide symbol output +; similarly, theoutputs of filters matched to w4, w5 and w10 will be ORed to providesymbol output −. The decoder thus will provide an output sequence of +and − symbols (each symbol being determined by which of the six matchedfilters generates the greatest output). A correlator will use a delayedversion of the transmitted primary sequence +++−−+− together with thedecoder output sequence to determine the value of cross-correlation.

As a result, the operation of correlation is performed on the primarysequence (with narrow autocorrelation) and its version reconstructedfrom received waveforms, utilizing the auxiliary information regardingthe waveforms used for random encoding.

The next composite pulse may then be sent with a different randomselection of waveforms.

It is anticipated that, in use of the present invention, the primarysymbol sequence will be generated in the form of a signal, and thensubjected to random encoding. However, this is not essential. Therandomly-selected waveforms could be produced directly, at theirrequired timings, without needing to generate the primary symbolsequence as a separate preliminary operation. (It would nevertheless bepossible to deduce for each waveform sequence a nominal primary symbolsequence, and its autocorrelation sequence, corresponding to the timingof the waveform generation.)

The obstacle-detection system of FIG. 5 may be mounted on a movableplatform (such as a vehicle or vessel), or on a stationary platform todetect the approach of a movable object. The system can be acollision-warning system arranged to generate a warning signal inresponse to detection of an object. Additionally or alternatively, thesystem may be a ranging aid for detecting the range of an obstacle andfor generating a signal indicative of the range.

It is desirable for the packets of transient signals generated bymultiple systems to satisfy the cross-correlation constraint mentionedabove, and in particular for the cross-correlation functions to havevalues which are all small compared with the maximum values of eachautocorrelation function. Furthermore, because of the desirability foreach system to have the same structure, it is preferable for theseconditions to apply to the cross-correlation properties of differentpackets produced by an individual system. This, however, can be achievedby virtue of the techniques described above, particularly the provisionof pulse mapping, random intervals between packets and timing sequencesselected to have good correlation properties.

It will be clear from the above description that references to the formof transient signals (e.g. single pulses or waveforms) are intended torelate to the baseband form of those signals; clearly the detailedstructure of transmitted waveforms may differ if the transient signalsare used for carrier modulation for transmission.

The term “random” is intended herein to include, without limitation, notonly purely random, non-deterministically generated signals, but alsopseudo-random and/or deterministic signals such as the output of a shiftregister arrangement provided with a feedback circuit as used in theprior art to generate pseudo-random binary signals, and chaotic signals.

The embodiments described herein can be implemented using dedicatedhardware, incorporating for example digital signal processors, or usingsuitably-programmed general-purpose computers.

Although some suitable formats of modulation, or waveform-for-pulsesubstitution, are currently regarded as preferable, other attributes ofwaveforms and wave phenomena (e.g., polarization of electromagneticwaves) can also be employed for ‘watermarking’ the interrogatingsignals.

The foregoing description of preferred embodiments of the invention hasbeen presented for the purpose of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. In light of the foregoing description, it is evidentthat many alterations, modifications, and variations will enable thoseskilled in the art to utilize the invention in various embodimentssuited to the particular use contemplated.

1-17. (canceled)
 18. A method of generating a sequence of waveforms,wherein: the waveforms are generated at timings corresponding to pulsesin a primary sequence having a narrow autocorrelation sequence, saidpulses being arranged in packets of predetermined configuration withgaps between the packets, and the timings of the pulses within eachpacket being such that each packet has a narrow autocorrelationsequence; and wherein each waveform is randomly selected from a set ofwaveforms with respective predetermined characteristics.
 19. A method asclaimed in claim 18, wherein successive pulse packets are randomlyselected from a predetermined set of pulse packets.
 20. A method asclaimed in claim 18, wherein the gaps between pulse packets are ofrandomly-selected length.
 21. A method as claimed in claim 18, whereinthe minimum gap between adjacent pulses in a packet exceeds apredetermined value, whereby the autocorrelation sequence of the packetexhibits a zero value for consecutive relative shifts which do notexceed a predetermined limit.
 22. A method as claimed in claim 18, inwhich the waveforms of said set are substantially mutually orthogonal.23. A method as claimed in claim 18, wherein the waveforms haverespective different frequencies.
 24. A method of generating a sequenceof waveforms, each waveform being generated at a liming corresponding tothat of a respective symbol in a primary sequence having a narrowautocorrelation sequence, the primary symbol sequence including symbolsof a plurality of types, wherein each waveform is randomly selected fromone of a plurality of sets of waveforms having respective predeterminedcharacteristics, the set from which the waveform is selected beingdependent on the type of the respective symbol.
 25. A method as claimedin claim 24, in which the primary symbol sequence comprises a firstpulse sequence interleaved with a second pulse sequence which is atime-reversed replica of the first pulse sequence, at least asubstantial number of the waveforms corresponding to each pulse sequencebeing distinguishable from those corresponding to the other pulsesequence.
 26. A method of generating a sequence of waveforms, thewaveforms being generated at timings corresponding to symbols in aprimary sequence having a narrow autocorrelation function, wherein eachwaveform is randomly selected from a set of waveforms with respectivepredetermined characteristics.
 27. A method of detecting an object, themethod comprising transmitting a sequence of waveforms generated using amethod as claimed in claim 26, receiving reflections of the transmittedwaveforms and determining matches between the transmitted and receivedwaveforms,
 28. A method as claimed in claim 27, wherein the transmittedwaveforms are selected from sets each corresponding to a respectivesymbol type in the primary symbol sequence, the method includingdecoding the received waveforms to obtain a received symbol sequence andcross-correlating the primary symbol sequence with the received symbolsequence to determine matches between the transmitted and receivedwaveforms.
 29. A method as claimed in claim 27, including the step ofstoring data indicating which waveforms have been randomly selected, andusing the stored data to determine matches between the transmitted andreceived waveforms.
 30. Apparatus for generating a sequence ofwaveforms, the apparatus being arranged to operate in accordance with amethod as claimed in
 18. 31. Obstacle-detection apparatus for use in amulti-user environment, the apparatus being arranged to operate inaccordance with a method as claimed in claim
 27. 32. Obstacle-detectionapparatus as claimed in claim 31, including means for providing a signalindicative of the range of a detected object.
 33. Obstacle-detectionapparatus as claimed in claim 31 for use in a vehicle or vessel todetect potential collisions.
 34. A collision-warning system for avehicle or vessel, the system comprising an obstacle-detection apparatusas claimed in claim 33 and means for generating a warning signal inresponse to obstacle detection.
 35. A ranging aid for a vehicle orvessel, the system comprising an obstacle-detection apparatus as claimedin claim 33 and means for generating a signal indicative of the range ofa detected obstacle.